Process control systems control industrial processes by means of various field devices connected to the process, such as regulating devices, control devices, transducers, transmitters, and the like. A typical field device is a control valve provided with a valve controller, such as the valve controller ND800 of Neles Automation. Devices known as intelligent field devices are equipped with control logic or software which allow for local control of the field device by means of a suitable control algorithm, for example, collection of both status and measurement data, and communication with an automation system or a specific field device management system by means of a field communication protocol, such as HART (Highway Addressable Remove Transducer). In addition, current intelligent field devices already comprise a sufficient amount of diagnostics to allow the field device to indicate when it malfunctions. This information can be utilized for focusing maintenance operations, which reduces unnecessary equipment testing and, thereby, the costs of maintenance. In addition, the utilization ratio of the plant (factory) increases as unpredictable down time is reduced.
A typical automation system comprises a control room with computers, databases, process control programs and user interfaces. There are various alternative ways to provide a connection between field devices and the rest of the system. Field devices are conventionally connected to the control system by two-wire twisted pair loops, each device being connected to the control system by a single twisted pair producing an analog 4 to 20 mA input signal. A process controller (PID) is arranged into a centralized computer system located in the control room. This type of process control system is often referred to as Direct Digital Control (DDC). In the next phase of control system evolution, a Distributed Control System (DCS) will be used, in which the process controllers (PID) are decentralized into a plural number of computers at the plant. The decentralized computers and the central computer located in the control room may be interconnected through a local data network or data bus, for example, whereas separate field devices remain connected to the process controllers through two-wire twisted pairs. Recently, new solutions have been adopted for the control systems, such as the Highway Addressable Remote Transducer (HART) protocol which allows digital data and a conventional analog 4 to 20 mA signal to be transmitted together in a twisted-pair loop. The most recent development phase involves a Field Control System (FCS) which employs a high-speed digital network or data bus for interconnecting the control room computer and the field devices. Conventional analog 4 to 20 mA signals have been omitted from the FCS, and a new communication protocol, commonly referred to as Fiedlbus, has been defined by the Instruments Society of America (ISA).
In principle, a field bus can be connected to any process device, thus allowing the devices to report their self-diagnostic data over the field bus to a maintenance computer, for example. However, all process devices do not support bus interfacing and self-diagnostic. For example, it has often not been necessary to connect devices such as pumps, mixers, refiners, screens, drums and switches to the field bus, although in some cases it would be useful to also monitor the diagnostics data of these devices in order to obtain timely information about their servicing needs, for example. To provide field bus cabling for these devices solely for this purpose would, however, often be a too high cost factor. Field bus cabling of dozens of meters to a device that is in a more remote location at the plant and the related mounting works may incur costs of thousands of dollars. In addition, in order for the device to be connected to and communicate with the field bus, it needs to be provided with I/O electronics. For example, Fieldbus typically requires a 16-bit processor and the related external electronics. The interface electronics involved also adds to costs considerably.
U.S. Pat. No. 5,793,963 teaches a control system comprising field devices which are connected to the control room with a Fieldbus cabling. In addition, some of the field devices are provided with a wireless Fieldbus gate through which a field device can be controlled over a wireless link using a portable control device or a workstation. The field device is thus provided with both a wireless and wired Fieldbus. The function of this wireless connection is to serve as a secondary, redundant control path, instead of a redundant, hardwired bus and to enable the field devices to be controlled directly by the service personnel using portable devices. This allows double cabling to be avoided. The wireless Fieldbus gate can use common interface electronics with the wired bus interface, and power supply to the field device can also be provided through the wired fieldbus.
The use of the described wireless Fieldbus gate without a wired Fieldbus would remove the above-mentioned cabling problem in diagnostics applications. Along with the cabling, also power supply to the interface and diagnostics electronics would be disposed of, and therefore power supply would have to be arranged locally. The power consumption of interface electronics is particularly high. But even if it were possible to arrange the power supply, the diagnostics and bus interface electronics would raise the price of the diagnostics unit to a considerably high level. The price would be too high in a case of several process devices, even though it would otherwise be interesting to automate their diagnosing.